Variable resistance



y 1937. J. 0. DE VRIES VARIABLE RESISTANCE Filed Oct. 5, 1954 W6 D J Patented May 25, 1937 UNITED STATES PATENT, OFFICE assignor to N. V. Philips Gloeilampenfabrieken', Eindhoven, Netherlands Application October 5, 1934, Serial No. 747,075 In the Netherlands November 4, 1933 2 Claims.

The term surface-grinding is to be understood to mean a type of grinding in which not only slight surface irregularities and a slight amount of the surface of the layer are removed, but also by which at the same time, by exerting pressure during the grinding operation, a rubbing together occurs. In the course of this grinding operation temperatures of 100 to 200 C. may

occur.

As a conductor in the dispersion, preferably carbon, for instance in the form oi graphite, is used,but one may also use materials such assilver oxide. Very suitable is the product sold under the registered trade-mark Aquadag, which consists essentially of colloidal graphite suspended in water.

Suitable resins capable of being hardened are for instance, shellac and artificial resins, for instance, phenolformaldehyde condensation products, capable of being hardened, the latter being preferably used.

The present invention has the advantage that 'a hard resistance layer can be obtained which makes it possible to use a spherically shaped contact body for making sliding contact therewith. Due to the hardness of the layer the sliding back and forth of this body during operation causes no excessive wearing of the resistance layer.

In fact, contact members having a comparatively large surface: for instance, in the form of a shoe were generally used hitherto with other resistance layers of this type for taking ofif current, in order that an excessive local pressure would not be exerted on the resistance layer and thus to avoid premature wearing out of said layer.

A contact that gives less rise to noises may be obtained, however, by using a contact body of a spherical type since in this case the contact point is exactly determined, the use of which is enabled by the invention.

It has already been suggested to give resistances a. definite value by polishing. Furthermore it is known per se to manufacture resistance layers by means of dispersions wherein a conductor and resin capable of being hardened are dispersed. However, the special advantages that can be achieved with the combination of these terial.

known operations were unknown up to the present.

The invention may be carried into effect by squirting the above-mentioned dispersion upon a solid insulating layer consisting of ceramic ma- Furthermore it is possible to impregnate paper or thin cardboard with the dispersion followed by drying, heating and grinding.

Preferably fibrous layers provided with a hardened resin are used, which layers are known under the registered; trade-marks ,Pertinax, Textolite, etc. As is well known these materials may be obtained by impregnating layers of fibers such as paper or tissues with a solution of an artificial resin capable of being hardened, followed by drying, heating and compressing.

This embodiment has the advantage that materials such as iPertinaX, Textolite and other similar materials are not only cheaper than insulating ceramic material, but can also be more readily manufactured in the form of a plate, and furthermore they already have, due to the pressing, a very smooth surface which is of great importance for the invention.

The invention allows this important embodiment since after application of the resistance layer at comparatively low temperatures, for instance, about 225 C. or less, one may proceed to baking in order that the organic insulating material be not attacked by high temperatures whereas the grinding operation permits the value of the reslstance'to be reduced exactly to the de-' sired point.

For manufacturing potentiometers the dispersion may be applied in such a manner that the resistance layer has a logarithmic course.

The invention will be more clearly understood by reference to the accompanying drawing.

Fig. 1 is a longitudinal section of apotentiometer, and

Fig. 2 is a bottom view thereof. 1

In Fig. l a ring I consisting of Pertinax is provided with a resistance layer 2 obtained by squirting an alcoholic solution of phenolformaldehyde condensation products capable of being hardened and preferably a pressure of about 150 gms. per square centimeter is exerted. The grinding operation may be followed by heating to about 200 C. for about 5 to 8 minutes in order to enhance the hardness.

The Pertinax plate I is secured to a supporting plate 3 consisting of insulating material, on which plate the contacts 4 and 5 are mounted at the same time. The spherical contact member l is electrically connected to the contact 5 and is elastically mounted and secured to a plate 8 consisting of insulating material. The described parts, as well as the knob In by means of which the shaft may be turned, are mounted on a shaft 9. Reference numeral H denotes a nut with the aid of which the plates 2 and 3 are fixed in the desired place. The spherical contact member I is preferably made of the adapted to slide'over said surface to take off the current.

2. A variable resistance comprising a resistance layer of a hardened resin and a conductive material, said layer having a rubbed smoothed work- -ing surface, and a spherically-shaped contact point of tombak adapted to slide over said surface to take off the current.

JOHAN DIRK- DE VRIES. 

